1. Field of the Invention
The invention relates generally to the removal of vapor phase contaminants from a gas stream. More specifically, the invention is directed to the removal of vapor phase mercury from the flue gas of a combustion system.
2. Description of Related Art
The emission of trace metals from utility power plants is an important issue in light of the 1990 Clean Air Act Amendment (CAAA), Title III. In particular, special attention has been given to mercury (Hg) in terms of its environmental release and impacts, and the Environmental Protection Agency (EPA) is closely scrutinizing sources that emit mercury. Mercury is present in flue gas from a fossil-fuel-fired combustion system in very low concentrations ( less than 1 ppm) and forms a number of volatile compounds that are difficult to remove. Specially designed and costly emissions-control systems are required to effectively capture these trace amounts of mercury.
Several approaches have previously been adopted for removing mercury from gas streams. These techniques include passing the gas stream through a fixed or fluidized sorbent bed or structure or using a wet scrubbing system. Approaches using fixed bed technologies normally pass the mercury containing gas through a bed consisting of sorbent particles or through various structures such as honeycombs, screens, or fibers that are coated with a sorbent. Common sorbents include activated carbon and noble metals such as gold and silver. In many cases where noble metals are used, the structure is coated with the noble metal sorbent while the support underneath is made of ceramic or metallic materials. The sorbents in these fixed structures can be periodically regenerated by heating the structure and driving off the adsorbed mercury (see, for example, U.S. Pat. Nos. 5,409,522 and 5,419,884, which are incorporated herein by reference). The mercury driven off can then be recovered or removed separately.
However, in regenerating the sorbent in such fixed bed systems, the bed must be taken off-line periodically. This necessitates that a second bed be used and remain online while the first one is regenerating. In addition, the beds need to be located downstream of a primary particulate collection device to remove all of the solid suspended particles in the gas stream and to avoid pluggage. These fixed bed systems also require significant space and are very difficult to retrofit into existing systems, such as into the ductwork of powerplants, without major modifications and high pressure drop penalites (e.g., 10-30 inches of water).
U.S. Pat. Nos. 5,948,143 and 6,136,072, which are incorporated herein by reference, described concepts that addressed some of these problems through the use of porous tubes and plates that can be regenerated and cleaned while in the presence of flue gas containing particles. These porous tubes and plates are cleaned by a series of back pulses across their walls. However, the fabrication of porous tubes and plates is complex and relatively expensive. The tubes and plates are also heavy and difficult to install and heat due to the thick wall requirements.
Therefore, a need remains for a cost-effective method of removing trace contaminants, in particular mercury, from the flue gas of a combustion system. Specifically, there is a need for a fixed bed process for removing such contaminants that can be easily retrofitted into an existing combustion process, easily cleaned to avoid plugging, and easily regenerated.
The present invention provides a method and apparatus for removing trace contaminants from a gas stream or flue gas. In one embodiment, the present invention an apparatus for removing a trace contaminant from a gas stream, comprising a gas duct configured to receive a gas stream comprising a trace contaminant; a plurality of substrates disposed within the gas duct; a trace contaminant sorbent disposed on at least a portion of each of the substrates; an isolation device for separating the portion of each of the substrates from the gas stream; and a regenerator for regenerating the trace contaminant sorbent.
In another embodiment, the present invention provides a method for removing a trace contaminant from a gas stream, comprising passing a gas stream comprising a trace contaminant through a gas duct; contacting said trace contaminant with a trace contaminant sorbent disposed on at least a portion of a substrate within said gas duct; adsorbing said trace contaminant with said trace contaminant sorbent; isolating said portion of said substrate from said gas stream; cleaning said substrate in-situ and concurrently with said passing of said gas stream through said gas duct; and regenerating said trace contaminant sorbent in-situ and concurrently with said passing of said gas stream through said gas duct.
The present invention provides a cost-effective method of removing trace contaminants, in particular mercury, from the flue gas of a combustion system. Specifically, the present invention provides a fixed bed process for removing such contaminants that can be easily retrofitted into an existing combustion process, easily cleaned to avoid plugging, and easily regenerated.
These and other features of the present invention will appear from the following description from which the preferred embodiments are set forth in detail in conjunction with the accompanying drawings.